Optical amplifiers are employed in the field of optical transmission technology for amplifying the optical signals transmitted in an optical network. The optical signals in many cases propagate over long links measuring several hundred kilometers and more in an optical fiber, being attenuated in the process. It is therefore necessary to amplify the optical signals when they have been transmitted over a long distance.
Optical links and networks of this type frequently employ Wavelength Division Multiplexing (WDM), a technique whereby a plurality of channels is transmitted in an optical fiber simultaneously at various wavelengths.
Erbium-doped fiber amplifiers (EDFAs) are largely employed in WDM transmission systems. An EDFA operates using an erbium-doped fiber into which the light from an optical pump, for example a laser diode, is coupled. The optical signal launched into the doped fiber is therein amplified by means of stimulated photon emission.
FIG. 1 shows a schematic representation of a conventional amplifier stage 10 of an optical amplifier that uses an erbium-doped fiber EDF 13. The amplifier stage further includes a WDM coupler 12 and an optical pumping device 14 whose light is coupled via the WDM coupler 12 into the doped fiber 13. The WDM signal (comprising, for instance, 80 channels) guided in the optical fiber 11 is amplified in the doped fiber 13 through stimulated emission. The amplifier gain is dependent on the pumping power of the pump 14 and is set by a control device (not shown) as required.
Addition and removal of individual channels of the WDM signal being transmitted on the fiber, component failures, fiber breaks or protection switching result in abrupt changes in power at the input of the amplifier. The pumping power of the optical pump has to be quickly matched to different input powers. The amplifier gain (defined as the output power/input power) would otherwise change and the output power of the individual signals would increase or decrease, as a consequence of which bit errors may occur at the receiver. Particularly in the case of multistage amplifiers, the deviations in gain in the individual stages can accumulate so that bit errors can very readily occur. A critical factor in the development of an optical amplifier is hence to maintain as constant as possible an amplifier gain even when large abrupt changes in power occur at the amplifier input.
FIGS. 2a-2d show the operation of a conventional amplifier stage 10 of an optical amplifier in an abrupt channel drop scenario and in particular the overshoot 23 and 24 which occurs immediately after a switching operation in the output signal Pout and in the gain G of an amplifier stage 10. FIG. 2a shows the curve of the input power Pin of an optical signal having, for example, 80 channels of equal power with a spacing of 50 GHz (0.4 nm) in the conventional wavelength band (C-band) that is being applied to the input of the amplifier stage. For example, at the instant t0, 79 of the 80 channels are removed from the amplifier, as a result of which the total power at the input Pin abruptly drops 20. FIG. 2b shows the curve of the pumping power Ppump. As can be seen, the pumping power Ppump is abruptly reduced 21 from the value Ppump|before (which keeps the gain G at a constant level 25 before the drop) to the value Ppump|after (which keeps the gain G at a constant level 26 after the drop) shortly after the instant t0 in response to the abrupt change 20 of the input power Pin. FIG. 2c shows the output power Pout of the amplifier stage, which likewise displays an abrupt change 22 approximately at the instant t0, which output power Pout likewise contains the overshoot 23 immediately after the instant t0. FIG. 2d shows the gain G of the amplifier stage 10, which gain is kept at a constant level before 25 and after 26 the drop and likewise contains the overshoot 24. There would be analogous undershooting if channels were added.
A large number of methods are known from the prior art whereby the amplifier gain can be kept substantially constant when there is a change in input power. One known method, for example, includes the use of a feedforward controller which measures the change in input power and, as a function thereof, calculates a new pumping power that will be set on the pump immediately thereafter. But there are typically some inaccuracies leading to permanent deviation from the target gain of the amplifier. The main difficulty with this lies in calculating the new pumping power correctly so that the amplifier gain will remain substantially constant. The pumping power is dependent not only on the amplifier's input power but also on the other channels' wavelength after the switching operation, and on other influencing variables. Basing the calculation of the pumping power requiring to be newly set solely on the change in power at the input is thus relatively imprecise.
Other known methods combine the feedforward controller with a feedback controller, which introduces over a longer period of time small modifications to the pump power level and thus helps to recover the original gain of the amplifier over time.
However, variations (overshooting or undershooting) in the amplifier gain will also occur after a switching operation even when the pumping power is optimally matched to a changed input power (which is to say is changed to the correct value in a single step). Said variations are due to the memory effect of the doping element in the fiber. The electrons in the doping element (erbium, for example) are, by means of optical pumps, first raised to a third higher energy level from which they drop, in a non-emitting state transition, to a lower metastable energy level. There will always still be many electrons at the third higher energy level when the pumping power is reduced abruptly, and these will later contribute to (undesired) intermediate gain variations. In particular, when using optical pumps with an emission wavelength around 980 nm, there is a theoretical limit for the minimum achievable deviations due to population of the third higher energy level. These intermediate gain variations can be detected as overshoots in the output power which, especially in the case of multistage amplifiers, can result in bit errors at the receiver through accumulation.
Based upon the above discussions, it is concluded that there is a need in the art, for an improved system and method for controlling an optical amplifier gain. The improved system and method should be capable of controlling an optical pump in such a fashion that the intermediate gain variations due to the memory effect of the doping element in the fiber are strongly reduced.